The present invention relates to sigma-delta beamformers. In particular, a sigma-delta beamformer with reduced demodulation error is provided.
A sigma-delta (or delta-sigma) beamformer converts analog signals received on each channel or for each transducer element into a digital bit stream. The converter produces a data stream with a small number of bits, such as a single bit for each sample, at a data rate much higher than the Nyquist sampling frequency of the input signal. The converter output is beamformed and converted to a more familiar multiple-bit data stream by filtering and decimation.
Beamforming techniques based on sigma-delta modulation (SDM) have been proposed to reduce the hardware complexity of conventional multi-bit digital delay-sum ultrasound beamformers. In conventional single-bit SDM beamforming, the single-bit SDM sequence for each channel is altered in accordance with the associated dynamic focusing delays, such as by inserting or dropping a bit value. The sum of all these altered sequences are then passed to a demodulator, which also serves as a decimator, to obtain the aimed focused signal. This whole process is equivalent to taking the sum of demodulation outputs for the altered (or distorted) SDM sequences. Due to this signal distortion for focusing, conventional single-bit dynamic focusing produces low SNR images. Moreover, the demodulator is a FIR lowpass filter that requires a few hundred multipliers in most cases.